Today computer-based systems can be found most everywhere and have made inroads into many devices that are used by society everyday, such as cell phones, handheld computers, automobiles, medical devices, personal computers and so on. In general, society has placed much reliance on computer-based systems to handle everyday tasks such as simple tasks like balancing checkbooks to relatively complex tasks such as predicting the weather. As technology improves, more and more tasks are migrated to computer-based systems. This, in turn, causes society to become more and more reliant on these systems.
A typical computer-based system comprises a system board and optionally one or more peripheral devices, such as display units, storage units and the like. The system board may contain one or more processors, a memory subsystem and other logic, such as serial device interfaces, network device controllers, hard disk controllers and the like.
The type of processors that are employed on a particular system board usually depends on the type of tasks performed by the system. For example, a system that performs a limited set of tasks, such as monitor emissions generated by an automobile engine and adjust an air/fuel mixture to ensure the engine is burning fuel completely may employ a simple specialized processor that is tailored to performing these tasks. On the other hand, a system that performs many different tasks, such as managing many users and running many different applications, may employ one or more complex processors that are general purpose in nature, configured to perform high-speed calculations and manipulate data to minimize the response time to servicing the users' requests.
The memory subsystem is a storage that holds information (e.g., instructions, data values) used by the processors. The memory subsystem typically comprises controller logic and one or more memory devices. The controller logic typically is configured to interface the memory devices with the processors and enable the processors to store and retrieve information to and from the memory devices. The memory devices hold the actual information.
Like the processors, the type of devices employed in a memory subsystem is often driven by the type of tasks performed by the computer system. For example, a computer system may have the task of having to boot without the assistance of a disk drive and execute a set of software routines that do not change often. Here, the memory subsystem may employ non-volatile devices, such as flash memory devices, to store the software routines. Other computer systems may execute very complex tasks that require a large high-speed data store to hold large portions of information. Here, the memory subsystem may employ high-speed high-density Dynamic Random Access Memory (DRAM) devices to store large portions of information.
Presently, hard disk drives have high densities that can store 20 to 40 Gigabytes of data, but are relatively bulky. However, flash memory, also known as solid-state drive, is popular because of its high density, non-volatility, and small size relative to hard disk drives. Flash memory technology is based on EPROM and EEPROM technologies. The term “flash” was chosen because a large number of memory cells could be erased at one time as distinguished from EEPROMs, where each byte was erased individually. The advent of multi-level cells (MLC) further increases Flash memory density relative to single level cells. Those of skill in the art will understand that Flash memory can be configured as NOR Flash or NAND Flash, with NAND Flash having higher density per given area due to its more compact memory array structure. For the purpose of further discussion, references to Flash memory should be understood as being either NOR or NAND or other type Flash memory.
Devices in a memory subsystem are often interconnected using a parallel interconnection scheme. This scheme involves interconnecting the devices in a manner such that address and data information and control signals are coupled to the devices in a parallel fashion. Each device may incorporate multiple inputs/outputs to accommodate the parallel transfer of the data and address information as well as control signals to the devices.